


You Can't Just Punch All Your Problems

by PitchBlackWings



Category: Undertale (Video Game), ワンパンマン | One-Punch Man
Genre: Also Known As Hell, Alternate Universes, Colliding Multiverses, Divergences From Canon, Fighting, Genos is such a Nerd, Infrequent Updates, Into Headcanon-Land, M/M, Metal Knight wants nothing to do with taking responsibility with destruction, Nightmare is Not Having Any Of It, No beta we die like Sans, Saitama is Not Having Any Of It Either, Saitama is much more intense/coolish here, You suffer with me
Language: English
Status: In-Progress
Published: 2020-12-05
Updated: 2021-01-28
Packaged: 2021-03-09 21:55:02
Rating: Not Rated
Warnings: Creator Chose Not To Use Archive Warnings, Graphic Depictions Of Violence
Chapters: 3
Words: 2,631
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/27863421
Author URL: https://archiveofourown.org/users/PitchBlackWings/pseuds/PitchBlackWings
Summary: Saitama has gotten used to the world as he knew it.  Sure, there were monsters everywhere, and the occasional alien invasion, but that wasnormalby now.  He knew exactly what to expect, and although it might be boring, he knew he could easily punch his way out of anything that could possibly come at him.Until the aliens-who-weren't-actually-aliens-they-actually-came-from-Earth-just-a-different-Earth come along, and Saitama is faced with a problem not even he can muscle his way out of.They're driving the Hero Association crazy, and everyone is panicking about the invasion that could mean the end of the world...But there's only six of them.  And they don't even want to be here.So guess who's in charge of watching out for them until they can find their way home again?This is gonna be fun.
Relationships: But there WILL be shipping - Relationship, Nothing at the moment bc it'd spoil the story heheh
Comments: 31
Kudos: 51





	1. Wasn't The Meteor Bad Enough? [Introductory/Prequel Chapter]

**Author's Note:**

> This whole idea came from a comment thread with Bookwyrm, and it invaded my brain until I was forced to write it down.
> 
> There will be extreme canon divergences because it's my altenate-universe now and I will shape and mold it to my will, muahaha!
> 
> God help us all.

**Summary for the Chapter:**

> Saitama just. Just. _Just._ Finished destroying a giant meteor threatening the city. Is it too much to ask for a short break to go shopping? 
> 
> Yep. It is, apparently.
> 
> Great.

**Notes for the Chapter:**

> The meteor incident made little sense to me in the manga, so I changed a few things. Saitama isn't going to be lazy and let a ton of huge rock chunks destroy the city, right?
> 
> ALSO while the story is written in English, the characters are speaking Japanese unless it's noted otherwise! ;3
> 
> Enjoy the chapter~

Saitama sighed.

This was just the most recent of a thread of threats to the various cities Saitama protected as a professional hero- a gigantic meteor that even Genos couldn't destroy, with all of his fancy gadgets and guns and the like. So, of course, his student had called for his help.

He really wanted to get this over with. It was Friday! Sale day in the marketplace! All the stores that sold his favorite sushi and rice balls were going to be having various sales. Saitama didn't make the most money, so Fridays were shopping days. He didn't trust Genos to shop within an inch of his life. The cyborg would likely spend hours on a single purchase, calculating exactly how he could save the most money.

Genos and some other guy were on a rooftop once Saitama had dashed to the meteor site- was that another hero? Metal Knight or something?

He mentally shrugged, gathering his legs underneath him and leaping into the sky, fist extended in a stereotypical superhero pose. There would be time to ask questions later.

As he neared the meteor, he began swinging his arms in quick rotations, so that when he hit the giant chunk of space rock-

_BANG-- BRAKA-BRAKA--BRRRKKK_

-The pieces of stone would be crushed and fractured into managable parts, instead of huge chunks destroying everything they crashed into. A rain of pebbles washed over the city underneath him, doing considerable damage, yes, but more like that of a bad hailstorm rather than a meteor shower.

Saitama landed heavily on the rooftop below, next to Genos, who was still staring at the storm of rock.

"Hey, Genos. What's up?" He turned to the large robot next to them. "Who are you?"

"I am Metal Knight." The mechanical creature turned its head to look at the dust filling the sky, and something in its gears rattled. "You seem to be capable. I must report my weapons test findings."  
Saitama huffed as Metal Knight flew- oh, it had rockets- off, leaving Genos and himself alone.

"Does he want us to take all the blame?" He hesitated. "Although technically he didn't do anything. So I guess the blame _is_ on me. That means angry people. Drat."

Genos was still silent, so Saitama walked closer, nudging him with an elbow. "Genos? Everything okay?"

The cyborg hesitasted. "My sensors are picking up an irregularity. Did you destroy all of the large chunks of rock, Master?"

"Huh?" Saitama thought for a bit. "Yeah, they were all pummeled to bits and parts. Smaller than my fist. Why?"

"There is a rather large box-shaped item in the middle of the dust storm on the floor below us," Genos explained, pointing. "And it's giving off heat signatures."

"Oh," Saitama said thoughtfully. "I'll check it out, then."

"There's no need to, Master," Genos quickly interjected, but Saitama was already gone, having jogged forwards and hopped off of the building, heading towards the middle of the rubble and stone. 

_All I need to do is check this weird box thing out, and then me and Genos can go shopping. There should still be sushi left..._

**Notes for the Chapter:**

> Next chapter should be coming hopefully soon but i mean who knows, not meeee ^^'
> 
> Saitama wants that 50%-off _sushi_


	2. Precussionary Maintenance

**Summary for the Chapter:**

> Precussionary Maintenance: 
> 
> The act of fixing something by using precussionary force; in layman's terms, hitting an object to fix it.

Saitama was picking through the rubble, Genos leading them towards the anomaly with his sensors.

Saitama probably could have found the thing himself, but Genos had insisted that it would take less time if he helped, so the cyborg was jogging through the street towards the object, Saitama close behind him.

Genos stopped in front of a very large slab of concrete and brick that had fallen off of the side of a nearby building onto the floor below. "My sensors indicate that the object is buried underneath here, Master. I would blast it away, but I don't know if it's a heat-sensitive technologic-"

Saitama sighed. "Twenty words, Genos."

The cyborg paused somewhat sheepishly.  
"…I might damage it if I use my cannons. Could you-"

Saitama picked up the edge of the concrete slab and heaved it away, wincing slightly as the street groaned and shuddered under the weight.

"…Is that it?"

Underneath the concrete that Saitama had just thrown away was a large cube, about forty feet by forty feet by his measurements. It was a dark black, but it seemed to almost pulse with light reflected off of the surface. It didn't seem to be scratched or dusty at all from the rubble around it, making it look almost as if it had been photoshopped into the scene.

"I believe so, Sensei," Genos replied slowly, circling the cube. "My readings are glitching out slightly. It seems to be made of obsidian, but it also seems to be giving off six distinct heat signatures of varying intensity."

"So there are six alive things inside of that thing?" Saitama squinted closer at it. It didn't look very impressive, or like it would be good at sustaining life. However, it had managed to survive the meteor in seemingly perfect condition, so maybe first glances weren't everything.

"Six heat signatures, Sensei," Genos corrected him, reaching out with a single finger to gingerly touch the surface of the cube. "Not necessarily living things. Just things that are giving off heat."

"Ah." Saitama reached out with a hand to touch the cube as well. "How do we open it?"

Genos frowned. "Let me try. It seems... well defended, however. I'm not entirely sure it CAN be opened."

The cyborg proceeded to use what seemed to be his blasters on a low, constant heat as welders, trying to burn through the surface of the cube.  
Saitama watched as genos struggled for about five minutes, getting more and more frustrated with the futility of his efforts.

"I… do not believe it was intended to be opened, Sensei," Genos said tiredly. "That, or I simply cannot figure out how to unlock it."

"Let me try," Saitama suggested, walking forwards and bringing up his hand parallel to the box. "Maybe I can get it to open."

"Sensei, wait! " Genos sprung back, worried. "I'm not sure that's a good-"

But it was too late.  
Saitama flicked his hand into the box at an angle, slapping the corner, and it began to shudder violently. He stepped back a few paces warily, eyeing it with mistrust.

Suddenly, the sides, each about five inches thick, fell apart, dragging the roof to the side and leaving the inside of the cube open to the eyes of the heroes watching.

Five small forms lay on the floor of what used to be the large, imposing cube, tangled in each other's arms and legs. In the center, a large machine stood, shaking with the occasional burst of electricity.

"Ah," Genos said quietly, seeming to be at a loss for words.

Saitama frowned, narrowing his eyes as he took a closer look at the contents of the cube before turning to his student.

"Why were six _skeletons_ giving off heat signatures, Genos?"


	3. Oi, Put The Lasers Away

**Summary for the Chapter:**

> Alternative Chapter Title: "Let Sleeping Skeletons Lie"
> 
> Waking up in the middle of a pile of rubble and black stone is no fun! And who is this bald dude, anyway?

**Notes for the Chapter:**

> WOOOOOO IM BACK WITH ANOTHER CHAPTER
> 
> my life is less of a mess now! ^^ And I hear the siren call of FANFICTIONNNN

Nightmare winced, stirring only slightly.

What had happened…?

He was no longer in the main hideout AU- the comforting aura of darkness was completely gone, making it evident that he had been moved. To his slight frustration, he couldn't think of what under Earth could have caused the sudden change in location.

He could feel the magical signatures of he others in the gang, though- Error's was naturally the strongest, but still wavering, as if he was on the edge of consciousness. The other four were entirely out, magic weak and watered down with exhaustion.

This was going to be a problem.

He could hear two voices, very nearby, cutting off his train of thought rather abruptly as he focused on listening to their conversation, still playing dead. The element of surprise would be a useful tool if it came to a fight.

"Sensei," a strange voice rang out, "I wouldn't touch them..."

Footsteps traipsed across a kind of stone, stopping right next to Nightmare's head. "They look like human skeletons," a new voice pointed out, sounding like they were right above him, "But also kinda weird. That one has weird glitchy things, and this one's all goopy. With tentacles."  
A finger poked at Nightmare's skull, and it took all his self control not to recoil in disgust.

"Sensei, I think the glitching one moved," The first voice fretted. "…I don't think they're dead."

"Really? Huh. This one looks pretty dead, though," the second voice- Sensei? Was that a name or a title? retreated slightly, kicking Nightmare in the ribs halfheartedly to prove its point. Nightmare seethed internally, choking down the urge to skewer whoever had dared to treat him as such.

"Why don't you break something? A bone or two on the glitchy one? If it was really moving, it's gotta react to that, right?" The second voice sounded completely nonchalant, as if it did this every day. "You can break bones pretty easily, right, Genos?"

"Of course, Master," the first voice- Genos- said, sounding slightly put-off. "I am well able to break bones without exerting my circuits."

Circuits? Was this Genos a robot or somthing? And they were going to hurt Error?

Nightmare made up his mind to make his move, gathering his strength and opening his eye, leaping to his feet and glaring at the offending creatures.

The second voice had belonged to a rather unimpressive man, bald as an egg, with a yellow and red suit-and-cape outfit. He reeked of LV, or something like it, as if he was no stranger to death and killing.

"Ah-"

Nightmare didn't let him finish his sentence, slamming a tendril into his toso and flinging him away into the rubble surrounding them. Without watching to see where he landed, Nightmare swung around frantically to the direction of Error's magical signature.

Standing above Error's prone body was a young man, metal implanted in his skin everywhere, with arms made entirely of robotic cybrnetics. His right arm was converted into some sort of gun, and it was aimed at Error's ribs. A bright white glow had started to flicker from the barrel of the weapon.

Nightmare summoned his magic for a teleport and blinked out of existence temporarily with a burst of exertion, appearing behind the robot-human and twisting his arm away with a tendril so that the energy blast wasn't aimed at anyone.

"What-!" The strange robot-human whirled around, startled, as Nightmare yanked him away from Error, twirling and tossing him away as he had done with the first human before kneeling at Error's side, lifting him up carefully. The glitch was still unconscious, only twitching every so often as a residual effect of some sort of aftershock.

This was bad.

Nightmare took in his surroundings quickly- nothing but rubble and a few buildings nearby, nothing that could help him right now. He might not have the energy to teleport himself and five people away at the same time, and he had no idea where he was. Or even what AU this was.  
And he was the only one awake...

A noise from behind him told Nightmare that the short break he had gained for himself was up. He swiveled around to see that the humans- both of them, what the heck?- were seemingly unharmed, and both were back, staring at him. The robot glared, while the bald one seemed almost apathetic.

Nightmare snarled, arcing his tendrils in a defensive position, positioning himself so that he was in front of all of his gang members, facing his opponents defiantly. "What do you want, humans?"

"What do _you_ want?" The bald one retorted, raising an eyebrow. "You're the monsters that just fell out of the sky in a meteor, after all."

"That's more rasict than usual, for a human," Nightmare said, narrowing his eye at them, crouching slightly. "What meteor?"

"You mean you don't know about-? All this?" The bald human gestured to the rubble around them. "You're monsters, didn't you do this? Isn't that what you do?"

Nightmare huffed. "We have Souls and lives just like you do. I don't know why I bother... do you have a hospital, at least, a Monster-friendly one?"

"No," the robot replied, voice harsh and brittle. "Why would we heal monsters when all they want to do is destroy humanity?"

"What do you-? You know what, never mind. Who are you?" Nightmare slowly allowed his tendrils to lower from above his head, trying to make himself seem less imposing.

"Nobody ever recognizes me," the bald one grumbled.

"I am the S-class hero Demon Cyborg," the robot supplied, voice clipped, "And this is my teacher, the Caped Baldy."

Nightmare frowned.  
"What are heroes supposed to do? Some sort of police or something?"

"We protect the people in the cities from monsters," Caped Baldy supplied. "The Hero Agency pays us."

"Wait a second, Sensei..." Demon Cyborg (what a dramatic name, wow) narrowed his eyes in guarded disbelief. "Did you not know who heroes are? Who is King? Amai Mask? Do you know Blast, or Bang? Anyone at all?"

"No, sadly, I've got no idea who or what those are," Nightmare grumbled. "I'm a bit new here, if you hadn't noticed. Did you see how I and my... fellows even got here?" He was stalling for time and fishing for information, both of which he needed desperately.

"You came out of a giant black box in a huge space rock that fell from the sky," The Caped Baldy supplied helpfully. "Does that mean you're aliens?"

"…I have no idea where this planet is," Nightmare admitted, shifting Error in his grasp as the glitching was beginning to make his bones numb in places. "Maybe I am, I don't know. You're sure you can't find us a healing Monster? Or a doctor, at least?"

"No," Demon Cyborg replied darkly. "No monster would ever help humanity by being a doctor. And all the hospitals would refuse to see a hostile monster. Or six."

"I'm not-" Nightmare bristled in agitation. He slipped his corruption-tentacles back into his spine, leaving him more exposed than he would like, despite being able to summon them back in a moment. "There, fine, i'm not 'hostile' anymore. You don't know anyone at all?"

"Not that would treat monsters, no," Caped Baldy replied, rocking back and forth on the balls of his feet. "Why do you need one? You don't look hurt."

Nightmare hissed. "My fam- my _subordinates_ are!"  
He turned his back on them purposefuly, lowering Error down to the ground and feeling his Soulbeat, resting a hand on his ribs. "If you can't help me, then leave us alone."

"We're supposed to kill monsters, though," The Caped Baldy pointed out, pointing at him. "And you look an awful lot like a monster."

Nightmare ripped a lower section of his hoddie off, shaking the excess corruption away before wrapping it quickly around Error's rib, setting a fracture. "...Monster with a capital M, ya mean. 'Cause that's what I am."

"Nah, we use lowercases," Caped Baldy said. "But it doesn't matter much, I guess. Genos, what do we do? He looks kinda friendly, in a mean way."

"It's probably an act, Sensei," Demon Cyborg- was Genos his real name?- replied. "And it did just admit to being a monster. I should be able to take it, now that I won't be caught off-guard."

"Fight me, and I'll rip that fancy- looking machinery right off your torso," Nightmare ground out, turning to face them yet again.

The Caped Baldy placed a hand on Genos's shoulder. "Eh, let me take care of it," he said, squeezing lightly before releasing the robot's arm and striding confidently forwards towards the skeletons. "We can still go shopping afterwards if I'm quick enough, right?"

Nightmare summoned his tentacles back, eyelight flicking from his unconscious family to the opponent advancing on him.  
What should he do....?

**Notes for the Chapter:**

> Fight scene? Fight scene coming???  
>  _perhaps_
> 
> I actually had a lot of fun writing this chapter! More than I have in a long time, to be honest ^^'
> 
> Kudos? Comments…? THEORY COMMENTS 0wO? ROLEPLAY COMMENTSsss? All are welcome! I enjoy interacting with you guys :3

**Author's Note:**

> Drop me a kudos or even maybe a comment if you liked it!


End file.
